The manufacture of semiconductors often requires dispensing various liquids on a silicon wafer. In Spin-On Glass (“SOG”) methods, a SOG material, typically a silicon dioxide solution, is dispensed by a nozzle onto the center of a silicon wafer. The wafer is then immediately rotated at a high speed, spreading the SOG material across the wafer. The amount of SOG material dispensed, surface tension of the SOG material solution, viscosity of the SOG material solution, the oxide concentration of the SOG material and the spin rate of the wafer affect the resulting film thickness.
In many semiconductor manufacturing systems, pumps and valves are used to control the amount of liquid dispensed from the nozzle. During the dispense process, a controller determines how much liquid has been dispensed based on the flow rate of the liquid and the amount of time the dispense process has been ongoing. When the appropriate amount of liquid has been dispensed, the controller can signal a control valve upstream of the nozzle to close, cutting off fluid flow to the nozzle. A suckback valve, also located upstream of the nozzle, can draw fluid remaining in the nozzle out of the nozzle.
In order to achieve proper uniformity of a SOG material layer across a wafer, the fluid must break off cleanly with no droplets hitting the wafer after the end of the dispense process. Many semiconductor manufacturing systems use open/close pneumatic valves to terminate a dispense process. An open/close valve will typically close with a single speed more quickly than desired to produce a clean break off. In other words, an open/close valve will typically slam shut when the controller signals the end of the dispense process. This can cause the fluid to severely oscillate at the end of the dispense process, potentially causing droplets or excess fluid to drip onto the wafer, thereby affecting the uniformity of film thickness on the wafer.
One solution that has been developed for this problem has been to employ proportional valves in which the rate of change of closure (i.e., the acceleration) can be set to a predefined value, such that the valve can close more slowly than “slamming shut.” One example of such a valve is a pneumatic control valve that uses a needle valve to control the pressure at the pneumatic control valve. Based on the state of the needle valve, the rate of closure of the pneumatic control valve is controlled. In these systems, a particular acceleration is selected and applied to the control valve such that rate of change of closure is substantially constant as the valve closes. While such systems can reduce droplets of excess fluid at the end of the dispense process, they can still allow some excess fluid to be deposited on the wafer.
Whether an open/close valve or proportional valve with predetermined rate of closure is employed, prior art semiconductor manufacturing systems suffer a further deficiency. After the control valve closes, a suckback valve is engaged that pulls remaining fluid up into the dispense nozzle. Drawing the fluid back into the nozzle too quickly can leave droplets in the nozzle. These droplets can crystallize, leading to problems in the next dispense process.